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Will a compression ring help with this coma?


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Hi all, 

With my current setup, CANON 600D + MPCC MKIII in a SW 200PDS with its stock 2" barrel (2 thumbscrews), I'm experiencing some residual coma (the image depicts the 4 corners and the center of a picture taken recently). 

Coma.png.8b336733968a24132c9b6d16913066f1.png

As you can see, the coma is more evident in the upper corners. 

I've tried a meticulous collimation (visual + laser) and measured the optical train backfocus (55mm according to Baader specs). 

Yesterday I tried turning the camera 180°, and it happened that the coma appeared at the bottom of the image. With this test, I think I can exonerate the MPCC.

Moreover, I realized that the MPCC doesn't fit exactly squared in the 2" barrel when tightening the 2 thumbscrews.

Now the questions: Could a small misalignment of the optical train cause this coma? Would a compression ring like this one, be helpful? Searching for the benefits of compression rings, some people in SGL and CN say that the compression ring do not center the optical train. However, I think that it is no a matter of centering but squaring. Any thoughts on this? Anyone with a comparable setup has experienced something similar?

Thank you very much in advance.

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The image suggests tilt in the optical train. Start by imaging a totally different location, in the sky. If the pattern changes, gravity may be to blame. Especially, if you image with the camera hanging down or pointing straight up, and the stars look good, it's gravity pulling on the camera. If the pattern doesn't change, it's definitely tilt. Fortunately, Skywatcher focusers have tilt adjustment screws. They are at the base of the focuser (the pairs of screws at the base in the image).

image.png.918e4f63bdad5cf863799addcfbd6017.png

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Thank you @wimvb , tilt makes much sense.

I've noticed the same pattern with different targets. And the optical train seems to be firmly hold. 

My suspicion is that the tilt is caused by the 2 thumbscrews when tightened. I could try inserting the MPCC in the barrel using Teflon tape instead of the thumbscrews. Then I would choose a target that allows the camera to be in a safe position and where gravity has less influence.

Makes sense?

EDIT: I've just came across these threads where the problem is tackled using a third thumbscrew:

 

Edited by barbulo
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Solved!! 
The coma was caused by a misalignment in the optical train. The MPCC’s barrel, when tightened with the 2 thumbscrews of the eyepiece holder (uneven pressure along the axis), lied unsquared in the focuser. Consequently, the camera’s sensor was not perpendicular to the light path. 
Temporary solution: gentle amount of Teflon tape in the CC barrel so it (and the camera) fits tightly enough and perfectly squared in the focuser. 
Permanent solution (still to come*): drill a third hole and screw-in a third thumbscrew in the eyepiece holder, 120º apart from the others.

This is the result of yesterday’s test:

750FC685-D3B8-4295-844D-BB8E7BF38E54.thumb.jpeg.c5ae813d7e88e67dd8349403316b7bf1.jpeg


* THIS IS EMBARRASSING: Yesterday I drilled the third hole in the focuser (turned out to be made of aluminum). When forcing the thumbscrew into the orifice (too narrow), it broke, leaving a piece inside the hole. Fortunately, the inner part of the ring is unobstructed, so the temporary solution is still available. Therefore, this “permanent solution” has been delayed until I find a new eyepiece holder and get it drilled by a profesional.

 

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